It has been traditionally practiced to nondestructively test structural members for material qualities, stresses, etc. by utilizing the dependence of the magnetic properties of materials on strain, structures such as crystal grain size and precipitates, etc. Such traditional methods include, for example, a method for estimating the tensile strength of steel by measuring its magnetic permeability and a method for estimating quenched hardness by measuring coercive force. In recent years, methods utilizing Barkhausen noise resulting from discontinuities in magnetization have been attracting attention, and there have been proposed a variety of methods utilizing this phenomenon; among them are a method of estimating the fatigue strength of soft steel (proposed, for example, by L. P. Karjalainen et al., IEEE Trans. Mag. MAG 16,514 (1980)) and a method of estimating the toughness of tool steel (proposed, for example, by Nakai et al., Iron and Steel, 75,833 (1989)).
To measure the magnetic permeability, coercive force, or Barkhausen noise of a body, an apparatus is used that comprises a power supply, a magnetization system consisting of a magnetizing head, etc., and a detection system consisting of a detection head, a signal processing system, etc. An advantage of the head system is that a body to be measured can be magnetized simply by touching the head to the surface of the body, for the detection of a signal from the body.
Using such magnetic techniques, attempts have been made to measure the stresses (axial forces) acting on a laid rail. Rails are constantly subjected to expansion and contraction due to changes in ambient temperatures, but usually the expansion and contraction of the rails are restrained except at portions near joints, since the rails are held rigidly to the crossties with fasteners. As a result, a compressive stress or a tensile stress acts locally on the rail. Since buckling may be caused in the rail when the compressive stress exceeds a certain critical value, it is particularly important in track maintenance to diagnose these axial forces. For this purpose, various techniques have been proposed for nondestructively detecting the axial forces acting on laid rails. For example, Japanese Unexamined Patent Publication No. 60-17330 discloses a rail axial-stress measuring apparatus which ensures reproducibility by demagnetizing the portions of a rail to be measured by a demagnetizer before measuring axial stresses using a magnetically anisotropic sensor. In this apparatus, the head and foot of a rail are taken as the portions to be measured. Further, Japanese Unexamined Patent Publication No. 60-243526 discloses a rail axial-stress measuring apparatus in which a pair of magnetically anisotropic sensors are arranged with their magnetically anisotropic detection coils differentially connected to eliminate the effects of external magnetic field disturbances. However, rails usually have residual stresses of varying magnitude locked into them before they were laid, and the magnitude of such stresses may be greater than the value of the axial forces, depending on the site within the rail. Using the apparatus disclosed in Japanese Unexamined Patent Publication No. 60-17330 or 60-243526, therefore, the absolute magnitude of the axial forces acting on the rail once it has been laid cannot be measured correctly unless the magnitude of residual stresses in the portion to be measured is obtained in advance.
The two prior art methods described above measure the change of permeability caused by stresses. Methods or apparatus for measuring stresses using other magnetic parameters are also disclosed, which include an apparatus for measuring stresses from the rate of change of coercive force (Japanese Unexamined Patent Publication No. 50-159787), a sensor, used in a stress and defect detection apparatus, for detecting Barkhausen noise by using a ferrite core having a rounded tip (Japanese Unexamined Patent Publication No. 60-57247), a method and apparatus in which measurement accuracy for stresses is enhanced by using Barkhausen noise in combination with acoustic emission (Japanese Unexamined Patent Publication No. 59-112257), and a method and apparatus for measuring stresses and mechanical properties by obtaining magnetic hysteresis curves using a flux meter (Japanese Unexamined Patent Publication No. 02-262026). However, none of the methods and apparatus disclosed above makes any mention of the measurement of axial forces acting on rails. Furthermore, by using any of these methods or apparatus, axial forces acting on a rail cannot be measured since the magnetic parameter of the rail composition itself changes little due to the axial forces. Moreover, by using any of these methods, the absolute magnitude of stresses cannot be obtained correctly because the values of the detected stresses are magnified by the residual stresses existing in the body to be measured.
As described above, when diagnosing axial forces acting on a laid rail by using magnetic techniques, with any prior art method or apparatus the absolute magnitude of axial forces acting on the laid rail cannot be obtained from the field measurements alone, because residual stresses were locked into the rail before it was laid. To obtain the absolute magnitude of such axial forces after the rail was laid, the prior art requires that the value of a magnetic signal induced by the residual stresses in the portion to be measured be measured and recorded in advance and be subtracted from the result of actual measurement. For a rail already laid, however, prerecording the values of the residual-stress-induced magnetic signals would not only be practically infeasible since there are an enormous number of portions to be measured, but it would also be difficult to align the magnetic head accurately with the same premeasured portion.
It is accordingly an object of the present invention to provide a rail axial-force measuring method which can measure axial forces acting on a rail quickly and accurately by measuring magnetic signals obtained from stress sensing portions provided on the rail and thereby eliminating the effects of residual stresses existing in the rail. It is another object of the invention to provide a rail that permits the measurement of axial forces acting on it.